KIDNEY
(Histology)
Learning Objective:
At the end of the lecture student will able:
To define the functional unit of the kidney.
To describe the different histological of the nephrons.
To describe the vasculature of the the kidney.
Kidneys
Kidneys are reddish brown paired organs, situated along the posterior abdominal wall.
They are covered by thin capsule, which is easily removed
The cut surface shows outer cortex and internal medulla.
Inside the medulla there is renal sinus containing cup shaped major and minor calycies
The renal medulla consists of pale, striated, conical renal pyramids.
Their bases are peripheral while their apices converging to the renal sinus where they project into minor
calyces as papilla.
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Each pyramid is capped by cortical tissue to form renal lobe.
The renal cortex is subcapsular; it also extends towards the renal sinus as renal columns.
The medullary tissue extends into the cortex as medullary rays.
Cortex close to the medulla is sometimes termed juxtamedullary cortex.
Microscopic Features
•
Kidney is composed of many tortuous closely packed uriniferous tubules, bound by little
connective tissue in which run blood , lymphatics and nerves.
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Each uriniferous tubule consists of two embryologically distinct parts.
1.
The secreting Nephrons which elaborate urine and
2. Collecting tubule which convey urine to the calyceal system.
Kidney and Urineferous Tubule
Nephron
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Each kidney consists of about 1,000,000 nephrons,
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The nephrons begins as double walled epithelial cup (Bowman’s capsule), which surrounds a tuft
of capillaries, the glomerulus.
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Bowman’s capsule and the glomerulus together form the renal or Malpigian corpuscle.
Bowman’s capsule consists of two simple epithelial layers; an inner or visceral layer covering the
glomerulus and an outer parietal layer. The region where the vessels enter or leave the glomerulus is
the ‘vascular pole of the corpuscle.
‘Urinary pole’ of the corpuscle is roughly opposite the vascular pole; here the parietal layer of
Bowman’s capsule becomes continuous with proximal tubule.
The Nephron and Renal Corpuscle
Malpigian corpuscle (Diagrammatic)
The cells of the visceral epithelium are specialized, called ‘podocytes’.
The nucleus and surrounding cytoplasm protrude into the capsular space and rests on the processes
that extend from the cell body, branching one or more times to form a series of slender processes on
basal lamina.
The terminal processes or pedicles interdigitate with the pedicles of neighboring podocytes; leaving a
series of slits between them.
Podocyte
Parietal Layer of Bowman’s capsule
Parietal layer of bowman’s capsule is lined by simple squamous epithelium.
This layer becomes continuous with the lining of proximal convoluted tubule.
Tubular Part of Nephron
The tubular portion of nephrons is composed of
A) proximal tubule
B) thin segment and
C) distal tubule.
Parts Of Nephrons
Both proximal and distal tubules have a convoluted portion and a straight portion.
The straight portion of the proximal tubule and distal tubule together with the thin segment are
arranged in a hairpin loop extending a variable distance into the medulla. These three segments
together are called ‘loop of Henle’.
The convoluted portion of the distal tubule joins the arched collecting tubule which begins system of the
ducts or ‘ collecting tubules’.
Proximal Tubule
The proximal tubule is the longest section of the nephron (about 14 mm).
The convoluted part of the proximal tubules coils close to the glomerulus in the cortex.
The diameter of proximal tubules is ~65 µm. Their walls are formed by a low columnar epithelium.
The eosinophilic cells of the epithelium have a wide brush border
They almost completely reabsorb substances of nutritional value from the glomerular filtrate (glucose,
amino acids, protein, vitamins etc).
In the proximal tubules the volume of the glomerular filtrate is reduced by about 75%.
Sodium ions are actively resorbed from the glomerular filtrate. They are followed by passively diffusing
chloride ions and the osmotic absorption of water.
The straight portion of the proximal tubule descends towards the medulla.
The straight portion of the proximal tubule merges with the intermediate tubule the thin segment of the
loop of Henle.
A flattened, only ~1-2 µm high epithelium (simple squamous) forms the intermediate tubule, which is
only ~15 µm wide.
Distal Tubule:
The diameter of the distal tubule expands to ~35 µm.
The distal tubule, is formed by low cuboidal cells without a brush border.
Basal infoldings and all the cell organelles are present.
Function of Distal convoluted tubule
Epithelial cells in distal tubules cells transport chloride (active) and sodium ions (passive) into the
surrounding peritubular space.
.
Cells in the distal tubules are sensitive to the hormone aldosterone.
Juxta-glomerular Complex
The distal tubule contacts the glomerulus forming a specialized section of tubular epithelium, the
macula densa.
At the point of contact with the glomerulus, the distal tubule is always in close contact with the efferent
and afferent arterioles of the glomerulus.
The juxtaglomerular cells surround the afferent arteriole (they are modified smooth muscle cells), which
produce and secrete renin.
Third component of the juxtaglomerular complex is extra-glomerular mesangial cells
JG Copmplex
Function of Renin
Renin activates angiotensinogen, a precursor found in the blood stream, leading to the formation of
angiotensin I.
Angiotensin I is converted to angiotensin II.
Angiotensin II is the most potent vasoconstrictor known.
Renin also stimulates the secretion of aldosterone
Blood supply of kidney
Renal artery is a branch of abdominal aorta.
Near the hilum it divides into anterior & posterior branches.
These branches further divide into segmental arteries apical upper, middle lower & posterior.
Renal vasculature
Renal artery
1. Interlobar arteries
2. Arcuate arteries
3. Interlobular arteries
4. Afferent arteriole
5. Glomerulus
6. Efferent arteriole
7. Vasa recta.
Cortical and medullary capillaries →interlobular vein →arcuate vein→renal vein
Renal vasculature